The present invention relates to a holding material for catalytic converter, for holding a catalyst carrier in a casing, and for use in a catalytic converter, for example, for purging exhaust gas emitted from an automobile or the like.
As known commonly, a catalytic converter for purging exhaust gas is mounted in a vehicle such as an automobile in order to remove emissions such as carbon monoxide, hydrocarbon and nitrogen oxides from exhaust gas emitted from an engine of the vehicle. Generally, as shown in FIG. 6 which is a sectional view, such a catalytic converter has a catalyst carrier 1 shaped like a cylinder, a metal casing 2 for receiving the catalyst carrier 1, and a holding material 3 interposed in a gap between the catalyst carrier 1 and the casing 2 while mounted on the catalyst carrier 1.
Generally, the catalyst carrier 1 has a cylindrical honey-comb molded material, for example, made of cordierite, and a precious metal catalyst carried by the molded material. It is therefore necessary that the holding material 3 interposed in a gap between the catalyst carrier 1 and the casing 2 has a function for holding the catalyst carrier 1 safely to prevent the catalyst carrier 1 from being damaged by collision with the casing 2 due to vibration or the like during the running of the automobile, and a function for sealing the catalyst carrier 1 to prevent non-purged exhaust gas from leaking out through the gap between the catalyst carrier 1 and the casing 2. Therefore, the holding material mainly used in the conventional art is a mat type holding material (e.g., see Japanese Application Publication Number 2002-66331 (JP2002-066331A)) of alumina fibers, mullite fibers or other ceramic fibers aggregated into a mat-like shape with a predetermined thickness, or a mold type holding material (e.g., see Japanese Application Publication Number Hei10-141052 (JP10-141052A)) molded into a cylindrical shape. Particularly the mold type holding material can be wound directly on the catalyst carrier 1, unlike the mat type holding material which has to be wound on the catalyst carrier 1 and supported by a tape or the like. Accordingly, the mold type holding material has an advantage to make it easy to produce the catalytic converter.
In order to obtain surface pressure necessary for holding the catalyst carrier 1, the holding material 3 is formed to have a basis weight (density) being not smaller than a fixed basis weight. Particularly in a diesel vehicle subject to rigid regulation of exhaust emission control, the catalyst carrier 1 is large in diameter, heavy in weight and high in exhaust pressure due to the influence of exhaust retarder. The holding material 3 is therefore requested to have a greater holding force. Thus, the holding material 3 is formed to have a considerably high basis weight.
Since the holding material 3 has inorganic fibers as its principal component, the gap between the fibers however nearly disappear when the basis weight of the holding material 3 increases. As a result, exhaust gas is blocked in the exhaust-gas-inlet-side end surface (e.g., a thick portion 3a on the left in FIG. 6) of the holding material 3. The exhaust gas contains plenty of acidic components such as NOx or SOx and flows in at a considerably high temperature and at a considerably high pressure. Thus, the exhaust-gas-inlet-side end surface 3a of the high basis weight holding material 3 potently suffers the wind erosion effect of the exhaust gas. As a result, the force that the holding material 3 has for holding the catalyst carrier 1 is lowered so that the catalyst carrier 1 is out of position. In the worst case, the catalyst carrier 1 may run into breakage.